Protection against transients

When the coil is disconnected from an electromagnet, peaks of inverse voltage appear at the terminals which can reach very high values.
These pulses can be transmitted down the line associated with the coil and could possibly affect other components.
In the case of a relay being operated by such devices as transistors, triacs, etc; it may be necessary to protect against transients.

Transients carried in the line

High voltage surges can be carried in the supply line to the relay coil.
These may appear in the form of peaks or bursts and are generated by the connection and disconnection of electric motors, transformers, capacitors etc.
Normally a relay is unaffected by these pulses, but if a diode is connected in association with the coil, it must be capable of withstanding an inverse voltage higher than those of the incoming peaks.

Protection circuits

A protection circuit must efficiently cope with pulses generated by the coil as well as incoming line surges (surges U1,2/50µs.)
Releco relays are available with integrated protection circuits or with modules plugged into sockets
S3-MP or S3-MS.

LED and protection circuit connected to coil:

Relays series MRC, QRC

LED consumption: 1 mA

X = LED indication with rectifier.
For DC and AC relays up to 250V.
Surges of 1000V up to 24V.
Surges of 2000V from 25 to 60V.
Surges of 4000V from 61 to 250V.
Note: LED connected in series with the coil @ 220Vdc in QRC types.

Increases release time approx. 4 times

D = Free-wheeling diode.

DX = Free-wheeling diode + LED indication.
Dampens transients caused by the relay coil on de-energisation.
Surges of 2000V up to 60Vdc.
Surges of 4000V from 61 to 250 Vdc.
Surges of 2000V in QRC types.

Increases release time approx. 4 times

F = Polarity and free-wheeling diodes.

FX = Polarity and free-wheeling diode + LED indication.
A diode in series with the coil protects the relay from reverse connection.
Surges of 1000V up to 60Vdc.
Surges of 4000V from 61 to 250 Vdc.
Surges of 2000V in QRC types.

Increases release time approx. 3 times

B = Bridge rectifier incorporated.

BX = Bridge rectifier and LED indication.
Allows the relay to operate in both AC or DC without any polarity inconvenience.
Available only in voltages up to 60V.
Surges of 1000V.

R = Resistor and capacitor.
Suppressor for AC coils.
Surges of 2000V.
Available only in MRC types.

Relays series IRC

X = LED indication with no polarity, (standard)
coils ≤ 12 V DC and AC.
LED rectifier bridge in parallel.

X = LED indication with no polarity, (standard)
coils ≥ 24V… DC and AC.
LED rectifiers bridge in series.

BX = LED indication with no polarity, (option)
only 24V and 48V AC/DC coils.
Rectifier bridge for AC/DC relays.

R = LED indication not available, (option)
RC protection against pulses on AC.

Protection against pulses

When a relay coil is disconnected, reverse voltage peaks may arise and reach very high values.
Said peaks can transmit to the coil associated line and other relays or semiconductors can be affected.

If triac, transistor, etc. controls a relay, appropriate steps must be taken to avoid or decrease peaks down to a non risky level.

Both polarity and free-wheeling diodes (FX), must protect coils, to avoid malfunctions, provided DC relays in battery are installed.

Making or breaking engines, transformers or contactors in an industrial environmental, may generate high voltage pulses, either isolated or burst, through the main line.
The voltage level of those pulse may be high enough to affect the isolation of the coil.

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